This invention relates to an inductor consisting of such an iron core made of a soft magnetic material, which serves as a magnetic circuit, around which is wound a coil, and more particularly to an inductor having a permanent magnet in part of the magnetic circuit thereof.
With an inductor for use in an electric circuit, through which overlapping A.C. and D.C. currents flow, as in the case with a choke coil, the iron core of the inductor is brought to a magnetic saturation due to D.C. electric current. For this reason, extremely low permeability of the iron core will result, and thus effective inductance of an inductor will be lowered.
To avoid this shortcoming experienced with the prior art inductor for use in an electric circuit, through which a great amount of D.C. current flows, there has been proposed a method, whereby to avoid the magnetic saturation in the iron core by providing a gap in part of the magnetic circuit thereof. However, the provision of a gap in part of the magnetic circuit will lead to the lowered permeability of the entire iron core which in turn results in lowered inductance of an inductor.
Another attempt to avoid the magnetic saturation in the iron core accruing from the D.C. current, which flows through an inductor, is that there is inserted in part of the magnetic circuit a permanent magnet having a magnetic field of an intensity of the same level as that of the D.C. magnetic field, with the direction of the magnetic field thereof being opposed to that of the D.C. magnetic field induced within the iron core due to a D.C. current. With the inductor of the aforesaid arrangement, the D.C. magnetic field induced within the iron core due to the D.C. current is off-set by the magnetic field produced by a permanent magnet, whereby there will not arise the magnetic saturation in the iron core made of a soft magnetic material.
This permits the iron core to operate at an extremely high level of permeability for an A.C. current, thereby providing a high inductance.
On the other hand, since not only the D.C. magnetic field an A.C. magnetic field are applied to the iron core of an inductor at the same time, materials having as high a specific resistance as possible are used as an iron core of a soft magnetic material, for instance, soft ferrites such as Mn-Zn ferrite and Ni-Zn ferrite and the like. The specific resistances of those materials are no less than 10.sup.2 .OMEGA.m. For the same reason, a hard ferrite magnet having a specific resistance of no less than 10.sup.2 .OMEGA.m, such as for instance, a Ba-ferrite magnet is used as permanent magnet to be inserted in part of the iron core of a soft magnetic material, of the inductor.
The coercive force of the hard ferrite magnet ranges from 2000 to 4000 oersted which is greater as compared with that an Alnico magnet, such that there will not be caused demagnetization by the use of the hard ferrite magnet for a portion, where A.C. magnetic field of a high level as well as a D.C. magnetic field having an opposed magnetizing direction are simultaneously applied as in the case with an iron core of an inductor.
On the other hand, the residual flux density of the hard ferrite magnet, as well known, is 3000 to 4000 gauss. Thus, the residual flux density thereof is half or one third as much as that of a metal magnet such as Alnico magnet. Accordingly, the use of the hard ferrite magnet as a biasing magnetic field source of an inductor is limited to the case where the D.C. component of the electric current is relatively small in amount. On the other hand, in case a D.C. component is relatively great, the D.C. magnetic field induced by the D.C. current will be much greater than that produced by the hard ferrite magnet, with the result that inductance of an inductor for the A.C. current will be lowered. For providing a higher level of biasing magnetic field by using an inductor which incorporates a hard ferrite magnet as a biasing magnetic field, it is required to use a magnet having a larger magnetic pole area. The use of a magnet having a larger magnetic pole area then dictates the use of a magnet of an increased length for avoiding demagnetization due to the demagnetizing field. (The thickness of the inserted magnet with respect to a direction of the magnetic circuit should be increased.) As has been described, with an inductor using as a biasing magnetic field a hard ferrite magnet which is inserted in part of a magnetic field, the size of the inductor should be increased, when an electric current having a great amount of D.C. current component is to flow therethrough.